The cornerstone is the first stone laid down in the construction of a building.
It is the reference point for every other stone.
The builders constantly measure against the stone, checking that the horizontal lines are square and true, and that the vertical lines are upright.
The slightest error can translate into instability and eventual failure of the entire structure.
God has given us the perfect cornerstone.
We can build our lives using Jesus Christ as our reference, and be assured it will be straight and true and upright.
Or we can use something else, and take our chances.